Test that trailing commas in object literals are allowed

ES6 will allow trailing commas in object literals.  It turns out that V8
already allowed it, too, as does JSC and SpiderMonkey.

R=marja@chromium.org
BUG=

Review URL: https://codereview.chromium.org/350353002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22005 ce2b1a6d-e550-0410-aec6-3dcde31c8c00